The traditional view of the complement system is that it is organized into three extracellular activation pathways that provide a powerful system for immunity against pathogens. During our previous granting period we discovered a novel additional innate pathway we term the intracellular complement system (ICS). Our discovery and characterization of the ICS was made while investigating the consequences of signaling through the complement regulator membrane cofactor protein (MCP/CD46) and centered initially on T cells. We found that complement can be activated not only on the cell surface (traditional view), but also intracellularly (new view). In a pilot study, T cells from patients wih autoimmune arthritis were found to have a hyperactive and dysfunctional ICS. Importantly, C3 was activated intracellularly to modulate T cell immunity via a convertase- independent mechanism mediated by cathepsin L to generate C3a. Intriguingly, this system is not restricted to T cells but has been identified in every cell examined to date indicating its widespread biologic significance. However, the mechanisms for generating C3a are unknown in other cell types. Our ongoing studies have expanded our knowledge and provided new understandings of the ICS. We have identified: a) a novel system for loading of inactivated (cleaved thioester bond) C3 [C3i or C3(H2O)] from plasma into the cell interior; b) intracellular stores in most cells of C4 and, selectively, of C5; and c) newly identified intracellular C3 ?-chain metabolic fragments. In sum, the unexpected but fortuitous finding of intracellular complement activation has opened the door for the discovery of how it impacts not only cellular homeostasis but also how hyperactivation or dysregulation of the ICS may impact autoimmune diseases. We hypothesize that an ICS is in steady-state equilibrium in most cells and becomes further engaged in the case of cell activation or injury. The system has the potential to generate C3a that binds to intracellular receptors and translocates to the cell surface and into the interstitiu. We propose to further investigate and define this unique and biologically relevant system with the following Specific Aims. 1) To elucidate the source, composition, metabolism and function of the ICS; 2) To characterize the response of the ICS to cellular activation, immune signaling and injury, and, 3) To characterize the ICS in autoimmunity. Relevance: We propose to focus on the newly discovered ICS as an additional pathway of complement activation and its dysfunction in autoimmunity. These studies have substantial significance for revealing novel targets for complement-based therapeutics.